1. Field of the Invention
The present invention relates to a method of and an apparatus for bending two overlapping glass plates simultaneously. Particularly, the present invention relates to a method of and apparatus for bending two overlapping glass plates which is suitable for deep-bending a side portion of the overlapping glass plates.
2. Description of the Background
A laminated glass is formed by laminating two glass plates and an intermediate plastic film such as polyvinyl butyral as an interlayer, and it is widely used for a front glass as a wind-shielding window for an automobile from the standpoint of safety.
In such laminated glass of this kind, it is necessary to bend two flat glass plates to thereby form a laminated glass because a curved laminated glass having the two-dimensional curved surface or the three-dimensional curved surface is required from the viewpoint of excellent design for automobiles. In this case, when the glass plates are separately bent, a delicate difference in shape appears between the glass plates to be laminated. Accordingly, when they are laminated with the intermediate layer interposed therebetween, such disadvantages as the failure to obtain a complete joint between the two glass plates, and the presence of air bubbles at the bonding surfaces which causes peeling off of the laminated glass result. Accordingly, a method of bending simultaneously two overlapping glass plates has been used for manufacturing laminated glass.
As a conventional method of bending glass plates for a laminated glass, there has been known a method in which a bending mold having a bend-shaping surface corresponding to a curved surface of the laminated glass is prepared; two glass plates are placed on the bending mold in an overlapping state; the bending mold is transferred into a heating furnace together with the glass plates; the glass plates are heated to a temperature capable of softening glass so that the glass plates are bent by their own deadweight as the glass plates are softened, whereby the glass plates are bent so as to correspond to the bend-shaping surface of the bending mold (Japanese Examined Patent Publication No. 10332/1974).
In such method, when it is necessary to deeply bend a side portion of the laminated glass, a bending mold having a fixed split mold in a ring form which has a bend-shaping surface corresponding to a generally curved portion (a gently curved portion) of the laminated glass and a movable split mold in a ring form which is placed at a side (or both sides) of the fixed split mold so that it can move to the fixed mold so as to be in alignment with the fixed mold by its own deadweight, and which has a bend-shaping surface corresponding to a portion to be deeply bent of a side portion of the laminated glass, is used. Two glass plates in a flat form are placed in an overlapping state on the bending mold with the movable split mold (molds) in a developed state; the bending mold is transferred into a heating furnace along with the glass plates; the glass plates are heated to a temperature capable of softening the glass plates; and as the glass plates are softened, the movable split mold is moved so as to come in alignment with the fixed mold, whereby the side portion of the glass plates is deeply bent by their deadweight by means of the movable split mold (Japanese Examined Patent Publication No. 1210/1973).
Recently, the demands of reducing the weight of an automobile and assuring the safety to passengers at the time of collision have been increasing, and a thin glass plate having a thickness of about 1.8 mm-2.5 mm has been used for a laminated glass which is used as a front windshield.
Moreover, a fitting structure called a flush mount system wherein a glass plate such as a front glass is fitted to the window frame of an automobile so that the surface of the car body in the vicinity of the window is substantially flush with the surface of the glass plate has been widely used. For the glass plate in the flush mount system, a high level of performance or quality such as strength is required from the standpoint of safety.
In view of the necessity of increasing the strength when the glass plates for a laminated glass are to be bent, it is necessary to cool the peripheral portion of the laminated glass at a predetermined cooling rate so that a compressive stress layer is formed at the peripheral portion of the laminated glass which is prepared by bending the glass plates in a softened state by applying heat. A technique which has been utilized along these lines involves placing glass plates on a bending mold so that the circumferential portion of the glass plates slightly projects from the bending mold; a deadweight bending method is carried out for the glass plates in a heating furnace, and the glass plates are introduced in a gradually cooling furnace to cool them at a predetermined cooling rate, whereby a requisite compressive stress layer is formed at the peripheral portion of the laminated glass plates.
However, when the thin glass plates as described above were bent by using the conventional bending method, the following problems occured. Namely, the glass plates were placed on the ring frame of the bending mold through its peripheral portion, and accordingly, it was difficult to sufficiently cool the peripheral portion of the glass plates in a gradually cooling furnace because the peripheral portions were in contact with the ring frame having a high thermal capacity. It was therefore, impossible to form a compressive stress layer in the peripheral portion of the glass plates, and a desired strengthening treatment could not be applied to the peripheral portion of the glass plates.
In order to eliminate the above-mentioned problem, a possible alternative is to apply a desired strenthening treatment to the peripheral portion in such as a manner that the peripheral portion of the glass plates projects from the ring frame of the bending mold; a bending operation is conducted under the condition that the preripheral portion is not in contact with the ring frame, and then the peripheral portion is gradually cooled. However, in the event a side portion of the glass plates is to be subjected to a deep-bending operation, it is necessary to locally press the side portion in addition to the above-mentioned bending operation. In this case, if the peripheral portion of the glass plates is projected from the ring frame of the bending mold, it is impossible to bend the glass plates into a desired shape, particularly, the peripheral portion into a desired shape.